Method and apparatus for constructing codebook, and method, apparatus and system for precoding

ABSTRACT

The present invention discloses a method and apparatus for constructing a codebook. The method comprises: receiving grouping information on a plurality of transmit antennas of a base station: obtaining a channel model for a channel between a mobile terminal and the base station; and determining an optimal precoding matrix corresponding to each antenna group according to the grouping information and the channel model, so as to construct a codebook used for precoding. The present invention further discloses a method and apparatus for selecting a precoding matrix, as well as a method, apparatus and system for precoding.

FIELD OF THE INVENTION

The present invention generally relates to precoding technology, andmore specifically to codebook construction, as well as a codebook-basedprecoding method, apparatus and system.

BACKGROUND OF THE INVENTION

Precoding technology is applied more and more widely in wirelesscommunication systems. Precoding technology is such that data is firstprecoded for channel loss compensation and then sent for better adaptingto channel conditions, whereby the data transmission quality and/orthroughput is improved.

At present, precoding technology can be divided into two categories.Take downlinks as an example. One category is such that a serving basestation of a mobile terminal obtains channel information on a downlinkmulti-input multi-output (MIMO) channel through pilot signals and so on,and then the serving base station calculates a precoding matrix andsends data subsequent to precoding the data. The other is acodebook-based precoding method wherein a mobile terminal obtainschannel information on a downlink MIMO channel through downlink pilots,preambles or midambles sent by a base station, then selects an index ofthe optimal precoding matrix from a codebook comprising a plurality ofprecoding matrixes and feeds the index back to a serving base stationthat sends data subsequent to precoding the data.

The issue of codebook construction is quite important in codebook-basedprecoding methods. There are currently two categories of codebookconstruction methods. One category is unstructured codebookconstruction, and the other is structured codebook construction.Generally speaking, a structured codebook is more popular in terms oflower construction complexity, better scalability and performances thanan unstructured codebook. Description is presented to the constructionof a structured codebook in the documents DFT based codebook (B. M.Hochwald, et al. ‘Systematic design of unitary space-timeconstellations’, IEEE Trans. Information Theory, Vol. 46, No. 6, 2000),Householder based codebook (IEEE C802.16e-04/527r4, ‘Improved feedbackfor MIMO precoding’, Intel, IEEE 802.16, Nov. 12, 2004) or Givenstransformation based codebook (R1-070728, ‘Proposed way forward oncodebook design for E-UTRA’. TI etc., 3GPP TSG RAN WG1 #48, February2007).

However, the complexity, scalability and performance of the structuredcodebook depend on the number of transmit antennas; the larger thenumber of transmit antennas or base stations, the worse the flexibilityof the structure codebook construction. Therefore, there is a need for abetter codebook construction method to adapt to the increasing number oftransmit antennas or base stations and precode data using the codebook.

SUMMARY OF THE INVENTION

In view of the foregoing problems, the present invention provides amethod and apparatus for constructing a codebook, as well as a method,apparatus and system for codebook-based precoding.

According to a first aspect of the present invention, there is provideda method for constructing a codebook, the method comprising the stepsof: receiving grouping information on a plurality of transmit antennasof a base station; obtaining a channel model for a channel between amobile terminal and the base station; and determining an optimalprecoding matrix corresponding to each antenna group according to thegrouping information and the channel model, so as to construct acodebook used for precoding.

According to a second aspect of the present invention, there is providedan apparatus for constructing a codebook, the apparatus comprising:receiving means for receiving grouping information on a plurality oftransmit antennas of a base station; channel model obtaining means forobtaining a channel model for a channel between a mobile terminal andthe base station; and precoding matrix determining means for determiningan optimal precoding matrix corresponding to each antenna groupaccording to the grouping information and the channel model, so as toconstruct a codebook used for precoding.

According to a third aspect of the present invention, there is provideda method for selecting a precoding matrix, the method comprising thesteps of: receiving grouping information on a plurality of transmitantennas of a base station; estimating a channel from the base stationto obtain a channel model; selecting from a codebook an optimalprecoding matrix corresponding to each antenna group, based on thegrouping information and the channel model; and sending an index of eachoptimal precoding matrix to the base station so that the base stationperforms concatenated precoding.

According to a fourth aspect of the present invention, there is providedan apparatus for selecting a precoding matrix, the apparatus comprising:receiving means for receiving grouping information on a plurality oftransmit antennas of a base station; channel model obtaining means forestimating a channel from the base station to obtain a channel model;precoding matrix selecting means for selecting from a codebook anoptimal precoding matrix corresponding to each antenna group, based onthe grouping information and the channel model; and sending means forsending an index of each optimal precoding matrix to the base station sothat the base station performs concatenated precoding.

According to a fifth aspect of the present invention, there is provideda method for precoding, the method comprising the steps of: constructinga concatenated precoding matrix according to indices of a plurality ofoptimal precoding matrixes received from a mobile terminal; precodingdata using the constructed concatenated precoding matrix; and sendingthe precoded data to the mobile terminal.

According to a sixth aspect of the present invention, there is providedan apparatus for precoding, the apparatus comprising: constructing meansfor constructing a concatenated precoding matrix according to indices ofa plurality of optimal precoding matrixes received from a mobileterminal; precoding means for precoding data using the constructedconcatenated precoding matrix; and sending means for sending theprecoded data to the mobile terminal.

According to a seventh aspect of the present invention, there isprovided a communication system, comprising an apparatus for selecting aprecoding matrix and an apparatus for precoding.

BRIEF DESCRIPTION OF THE DRAWINGS

The other objects and effects of the present invention will become moreapparent and easier to understand from the following description ofspecific embodiments illustrating principles of the present invention,when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of an apparatus for constructing a codebookaccording to the present invention;

FIG. 2 is a flowchart of a method for constructing a codebook accordingto the present invention;

FIG. 3 is a block diagram of an apparatus for selecting a precodingmatrix according to the present invention;

FIG. 4 is a flowchart of a method for selecting a precoding matrixaccording to the present invention;

FIG. 5 is a block diagram of an apparatus for precoding according to thepresent invention;

FIG. 6 is a flowchart of a method for precoding according to the presentinvention;

FIG. 7 illustrates a comparison between simulation results ofconcatenated precoding according to the present invention, idealprecoding and existing precoding technology; and

FIG. 8 illustrates a comparison between simulation results ofconcatenated precoding according to the present invention, phase and/oramplitude weighted concatenated precoding according to the presentinvention, ideal precoding and existing precoding technology.

The same reference numerals denote like, similar or correspondingfeatures or functions throughout the foregoing figures.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is now explained and illustrated in more detailwith reference to the accompanying drawings. It should be understoodthat the figures and embodiments of the present invention are only forillustration purposes and not for limiting to protection scope of thepresent invention.

In the present invention, a codebook is a set of precoding matrixes. Inthe codebook, each precoding matrix corresponds to an index via which acorresponding precoding matrix is found. The codebook may be prestoredin a mobile station and a base station according to standards or sent toa mobile station or a base station by signaling.

For illustration purposes, in specific embodiments of the presentinvention, in single-BS MIMO a base station has 4 transmit antennas,while in multi-BS MIMO there are 2 base stations each having 2 transmitantennas, and a mobile terminal has 2 receive antennas. A channel modelfrom the base station to the mobile terminal is H. In multi-BS MIMO, forthe case where more than two base stations jointly serve one mobilestation, it is well within reach of those skilled in the art by simplyextending the following embodiment according to the present invention.It should be pointed out that the two base stations used in theembodiment of multi-BS MIMO of the present invention are only forillustration purposes and should not be construed as limiting thepresent invention.

FIG. 1 is a block diagram of an apparatus for constructing a codebookaccording to the present invention. In FIG. 1, an apparatus 100determines a precoding matrix corresponding to each transmit antennagroup according to grouping information on transmit antennas of a basestation and a channel model, and constructs a codebook using thedetermined precoding matrix. Specifically, the apparatus 100 comprises:receiving means 110, channel model obtaining means 120 and precodingmatrix determining means 130. Among them, the receiving means 110 isconfigured for receiving grouping information on a plurality of transmitantennas of the base station; the channel model obtaining means 120 isconfigured for obtaining a channel model for a channel between a mobilestation and the base station; the precoding matrix determining means 130is configured for determining a precoding matrix corresponding to eachtransmit antenna group according to the grouping information and thechannel model, so as to construct a codebook used for precoding.

In an embodiment, the precoding matrix determining means 130 furthercomprises means for applying the grouping information and the channelmodel to predetermined criteria so as to determine the optimal precodingmatrix that satisfies the predetermined criteria.

In another embodiment, the precoding matrix determining means 130further comprises: means for calculating, according to the groupinginformation and the channel model, a precoding matrix that makes aChordal distance, a projection two-norm distance or a Fubini-Studydistance be minimum; and means for determining the precoding matrix thatmakes the distance be minimum as the optimal precoding matrix.

FIG. 2 illustrates a flowchart of a method for constructing a codebook.It should be pointed out that steps in FIG. 2 may be performed by thecorresponding means in FIG. 1, respectively.

In step 201, grouping information on a plurality of transmit antennas ofa base station is received.

The method for constructing a codebook of the present invention may beapplied to both single-BS MIMO and multi-BS MIMO. In this embodiment,single-BS MIMO with 4 transmit antennas is assumed. Besides, it isassumed that the 4 transmit antennas of the base station is divided into2 groups each having 2 antennas. It should be pointed out that in actualapplications, the method of the present invention is not limited to onlya single base station with 4 antennas but may be applied to a singlebase station with more transmit antennas or to multiple base stations.

In an embodiment, the grouping information includes the number of basestations in the MIMO system, the number of transmit antennas at eachbase station, the number of groups resulting from grouping the antennasof the base station, the number of antennas in each group, etc.

In the present invention, the antennas of the base station are groupedin various manners. For example, when the system comprises only one basestation and the base station has 4 transmit antennas, the antennas maybe equally divided into 2 groups each having 2 antennas; at this point,the antennas may also be unequally divided into 2 groups, one of whichhas 3 antennas while the other of which has only 1 antenna. When thesystem comprises a plurality of base statioins, 2 BSs for example, andeach base station has 2 transmit antennas, 2 antennas at each basestation may be divided into a group whereby 2 antenna groups areobtained; at this point, one of antennas at each base station may betaken as a group whereby 2 antenna groups are obtained.

In the present invention, various manners of grouping antennas exist andare feasible so long as the number of antennas of each group is not toolarge. Only by making the number of antennas of each group less than thetotal number of antennas, is it possible to reduce the size of acorresponding precoding matrix and further improve the flexibility ofprecoding.

The grouping information is stored in a hard disk or a removable memorylike an optical disk or a floppy disk, downloaded via the Internet orother computer, or determined in real time by an apparatus duringexecution of the method illustrated in FIG. 2.

In step 202, a channel model for a channel between a mobile terminal andthe base station is obtained.

In the embodiments of the present invention, the channel model isprestored in a hard disk or memory like an optical disk or a floppydisk, or is downloaded via the Internet or other computer.

In an embodiment, the channel model is represented using a channelmatrix H. Now that in the prior art there are a variety of methods forestimating a channel model, those skilled in the art may easily obtain achannel matrix according to the prior art, and thus details are omittedhere.

In step 203, the optimal precoding matrix corresponding to each antennagroup is determined according to the grouping information obtained instep 201 and the channel model obtained in step 202, so as to constructa codebook used for precoding.

In an embodiment, during the process of determining the optimalprecoding matrix corresponding to each antenna group, the groupinginformation and the channel model are applied to predetermined criteriato determine the optimal precoding matrix that satisfies thepredetermined criteria.

In an embodiment, the process of determining the optimal precodingmatrix corresponding to each antenna group is described as below:calculating, according to the grouping information and the channelmodel, a precoding matrix that makes a Chordal distance, a projectiontwo-norm distance or a Fubini-Study distance be minimum, and determiningthe precoding matrix that makes the distance be minimum as the optimalprecoding matrix.

In an embodiment, the predetermined criteria are selected depending onreceiver architecture (Minimum Mean Square Error MMSE, MaximumLikelihood ML, etc.) and performance metric (capacity, bit error rate,etc.) (see D. J. Love, et al. ‘Limited feedback unitary precoding forspatial multiplexing systems’, IEEE Trans. On Information Theory, Vol.51, No. 8, 2005).

In an embodiment, the predetermined criteria are selected to minimizethe Euclidian distance (equivalent to maximizing the minimum singularvalue). When MMSE detecting is used at a receiver, first a group ofprecoding matrixes corresponding to respective antenna groups is set(the index of such precoding matrixes is equal to that of antennagroups), a concatenated precoding matrix is obtained according to therespective precoding matrixes, and a product matrix of the channelmatrix and the concatenated precoding matrix is calculated to obtain theminimum singular value of the product matrix; then, precoding matrixescorresponding to respective antenna groups are reset, the foregoingprocess is repeated, a plurality of minimum singular values resultingfrom times of setting are compared to determine the maximum one of theplurality of minimum singular values, and finally a precoding matrixcorresponding to the maximum minimum singular value is obtained as theoptimal precoding matrix.

Expression

$\underset{{{\hat{W}}^{m} \in \hat{W}},{{\hat{W}}^{n} \in \hat{W}}}{argmax}\lambda_{\min}\left\{ {H\overset{\sim}{W}} \right\}$describes the foregoing process, where λ_(min){•} is an operatorcalculating the minimum singular value, Ŵ^(m) and Ŵ^(n) represent twoprecoding matrixes, {tilde over (W)} represents the concatenatedprecoding matrix obtained according to Ŵ^(m) and Ŵ^(n), and H is amatrix representing the channel model.

In an embodiment, the concatenated precoding matrix obtained fromrespective precoding matrixes is achieved by directly combining therespective precoding matrixes into a large precoding matrix. Forexample, when there are 2 antenna groups, precoding matrixescorresponding to the antenna groups also total 2, e.g., represented byŴ¹ and Ŵ² respectively, where

${{\hat{W}}^{1} = \begin{bmatrix}{\hat{w}}_{1}^{1} \\{\hat{w}}_{2}^{1} \\\vdots \\{\hat{w}}_{n}^{1}\end{bmatrix}},{{\hat{W}}^{2} = \begin{bmatrix}{\hat{w}}_{1}^{2} \\{\hat{w}}_{2}^{2} \\\vdots \\{\hat{w}}_{m}^{2}\end{bmatrix}},$

and combination of Ŵ¹ and Ŵ² is

$\overset{\sim}{W} = {\begin{bmatrix}{\hat{w}}_{1}^{1} \\{\hat{w}}_{2}^{1} \\\vdots \\{\hat{w}}_{n}^{1} \\{\hat{w}}_{1}^{2} \\{\hat{w}}_{2}^{2} \\\vdots \\{\hat{w}}_{m}^{2}\end{bmatrix}.}$

{tilde over (W)} shown above is the concatenated precoding matrixobtained according to Ŵ¹ and Ŵ². In an embodiment, the concatenatedprecoding matrix {tilde over (W)} is weighted to satisfy thenormalization requirement, e.g., multiplying the concatenated precodingmatrix {tilde over (W)} by a weighting coefficient √{square root over(2)}/2. In another embodiment, the plurality of precoding matrixes Ŵ¹and Ŵ² are weighted respectively, and then the plurality of weightedprecoding matrixes are combined into a concatenated precoding matrix.

By means of the method illustrated in FIG. 2, a codebook containingprecoding matrixes is obtained, where each precoding matrix correspondsto a unique index. In this codebook, a corresponding precoding matrixcan be found according to the index. In an embodiment, the codebook isstored respectively in the base station and the mobile terminal of thecommunication system so that the base station and the mobile terminalcan synchronously select precoding matrixes. For example, when themobile terminal sends to the base station an index of a precodingmatrix, the base station can find a codebook according to the index andthus determine which precoding matrix the mobile terminal wants the basestation to use.

In an embodiment, the codebook obtained according to the codebookconstruction method as illustrated in FIG. 2 is respectively stored inthe mobile terminal and the base station, so that the mobile terminaland the base station have the same codebook and can synchronously selectprecoding matrixes.

In the present invention, the apparatus illustrated in FIG. 1 and forexecuting the codebook construction method of FIG. 2 may be a mobileterminal, abuse station, or other server in a communication system, etc.

If the apparatus is a mobile terminal, the mobile terminal stores acodebook in itself while constructing the codebook, and sends theconstructed codebook to a base station so that the mobile terminal andthe base station have the same codebook.

If the apparatus is a base station, the base station stores a codebookin itself while constructing the codebook, and sends the constructedcodebook to a mobile terminal so that the base station and the mobileterminal have the same codebook.

If the apparatus is a server in a communication system other than amobile terminal and a base station, a codebook constructed by the serveris respectively sent to a mobile terminal and a base station so that themobile terminal and the base station have the same codebook.

FIG. 3 illustrates a block diagram of an apparatus 300 for selecting aprecoding matrix according to the present invention. The apparatus 300comprises: receiving means 310, channel model obtaining means 320,precoding matrix selecting means 330 and sending means 340. Among them,the receiving means 310 is for receiving grouping information on aplurality of transmit antennas of a base station; the channel modelobtaining means 320 is for estimating a channel from the base station onas to obtain a channel model; the precoding matrix selecting means 330is for selecting from a codebook the optimal precoding matrixcorresponding to each antenna group, based on the channel model; thesending means 340 is for sending an index of each optimal precodingmatrix to the base station so that the base station can performconcatenated precoding.

In an embodiment, the precoding matrix selecting means 330 furthercomprises: means for applying the grouping information and the channelmodel to predetermined criteria so as to determine a precoding matrixthat satisfies the predetermined criteria; and means for retrieving inthe codebook a precoding matrix that matches the precoding matrixsatisfying the predetermined criteria, as the optimal precoding matrix.

In an embodiment, the means comprised in the precoding matrix selectingmeans 330 and for applying the grouping information and the channelmodel to predetermined criteria so as to determine a precoding matrixthat satisfies the predetermined criteria further comprises: means forcalculating, according to the grouping information and the channelmodel, a precoding matrix that makes a Chordal distance, a projectiontwo-norm distance or a Fubini-Study distance be minimum; and means fordetermining the precoding matrix that makes the distance be minimum asthe optimal precoding matrix.

In an embodiment, the apparatus illustrated in FIG. 3 and for selectinga precoding matrix according to the present invention is a mobileterminal.

FIG. 4 illustrates a flowchart of a method for selecting a precodingmatrix according to the present invention. It should be pointed out thatsteps in FIG. 4 may be performed by the corresponding means in FIG. 3,respectively.

In step 401, grouping information on a plurality of transmit antennas ofa base station is received.

The method for constructing a codebook of the present invention may beapplied to both single-BS MIMO and multi-BS MIMO. In this embodiment,single-BS MIMO with 4 transmit antennas is assumed. Besides, it isassumed that the 4 transmit antennas of the base station is divided into2 groups each having 2 antennas. It should be pointed out that in realapplications, the method of the present invention is not limited to onlya single base station with 4 antennas but may be applied to a singlebase station with more transmit antennas or to multiple base stations.

In an embodiment, the grouping information includes the number of basestations in the MIMO system, the number of transmit antennas at eachbase station, the number of groups resulting from dividing the antennasof the base station, and the number of antennas in each group.

In an embodiment, the antennas of the base station are grouped invarious manners. For example, when the system comprises only one basestation and the base station has 4 transmit antennas, the antennas maybe equally divided into 2 groups each having 2 antennas; at this point,the antennas may also be unequally divided into 2 groups, one of whichhas 3 antennas while the other of which has only 1 antenna. When thesystem comprises a plurality of base stations, 2 BSs for example, andeach base station has 2 transmit antennas, 2 antennas at each basestation may be divided into a group whereby 2 antenna groups areobtained; at this point, one of antennas at each base station may betaken as a group whereby 2 antenna groups are obtained.

In the present invention, various manners of grouping antennas exist andare feasible so long as the number of antennas of each group is not toolarge. Only by making the number of antennas of each group less than thetotal number of antennas, is it possible to reduce the size of acorresponding precoding matrix and further improve the flexibility ofprecoding.

In the present invention, the grouping information is stored in a harddisk or a removable memory like an optical disk or a floppy disk,downloaded via the Internet or other computer, or determined in realtime by an apparatus during execution of the method illustrated in FIG.4.

In step 402, a channel from the base station is estimated to obtain achannel model.

In an embodiment, the channel model is represented using a channelmatrix H. Now that in the prior art there are a variety of methods forestimating a channel model, those skilled in the art may easily obtain achannel matrix according to the prior art, and thus details are omittedhere.

In step 403, the optimal precoding matrix corresponding to each antennagroup is selected from a codebook based on the grouping informationobtained in step 401 and the channel model obtained in step 402.

In an embodiment, the grouping information and the channel model areapplied to predetermined criteria to determine a precoding matrix thatsatisfies the predetermined criteria.

In an embodiment, a precoding matrix that makes a Chordal distance, aprojection two-norm distance or a Fubini-Study distance be minimum iscalculated according to the grouping information and the channel model,and subsequently the precoding matrix that makes the distance be minimumis determined as a precoding matrix that satisfies the predeterminedcriteria.

In an embodiment, the predetermined criteria are selected depending onreceiver architecture (Minimum Mean Square Error MMSE, MaximumLikelihood ML, etc.) and performance metric (capacity, bit error rate,etc.).

In an embodiment, assume that MMSE detecting is used at a receiver.First of all, a group of precoding matrixes corresponding to respectiveantenna groups is set, a concatenated precoding matrix is obtainedaccording to the respective precoding matrixes, and a product matrix ofthe channel matrix and the concatenated precoding matrix is calculatedto obtain the minimum singular value of the product matrix, then,precoding matrixes corresponding to respective antenna groups are reset,the foregoing process is repeated, a plurality of minimum singularvalues resulting from times of setting are compared to determine themaximum one of the plurality of minimum singular values, and finally aprecoding matrix corresponding to the maximum minimum singular value isobtained as the optimal precoding matrix.

In an embodiment, the concatenated precoding matrix obtained fromrespective precoding matrixes is achieved by directly combining therespective precoding matrixes into a large precoding matrix. In anotherembodiment, the to concatenated precoding matrix is weighted to meet thenormalization requirement. In another embodiment, the plurality ofprecoding matrixes are weighted respectively, and then the plurality ofweighted precoding matrixes are combined into a concatenated precodingmatrix.

Then, a precoding matrix that matches the precoding matrix satisfyingthe predetermined criteria is retrieved in a codebook as the optimalprecoding matrix. In an embodiment, by comparing the precoding matrixsatisfying the predetermined criteria with each of precoding matrixescontained in the codebook, a matrix with the minimum difference isdetermined and used as a precoding matrix that matches the precodingmatrix satisfying the predetermined criteria. In the meanwhile, an indexcorresponding to the matching precoding matrix is determined.

In an embodiment, the codebook is prestored in a base station and amobile terminal and the codebook is completely identical. The codebookmay be obtained by the method illustrated in FIG. 2 or preset by a user.

In step 404, the index of the optimal precoding matrix is sent to thebase station an that the base station can perform concatenatedprecoding.

FIG. 5 illustrates a block diagram of an apparatus 500 for precodingaccording to the present invention. The apparatus 500 comprises:constructing means 510, precoding means 520 and sending means 530. Amongthem, the constructing means 510 is for constructing a concatenatedprecoding matrix according to indices of a plurality of optimalprecoding matrixes received from a mobile terminal; the precoding means520 is for precoding data using the constructed concatenated precodingmatrix; and the sending means 530 is for senting the precoded data tothe mobile terminal.

In an embodiment, the constructing means 510 further comprises: meansfor retrieving in a codebook precoding matrixes that correspond to theindices of the plurality of optimal precoding matrixes; and means forcombining the plurality of retrieved precoding matrixes into theconcatenated precoding matrix.

In an embodiment, the means for combining the plurality of retrievedprecoding matrixes into the concatenated precoding matrix furthercomprises: means for respectively weighting the plurality of retrievedprecoding matrixes; and means for combining the plurality of weightedprecoding matrixes into the concatenated precoding matrix.

In an embodiment, the means for combining the plurality of retrievedprecoding matrixes into the concatenated precoding matrix furthercomprises: means for weighting the concatenated precoding matrix toachieve normalization.

In an embodiment, the apparatus for precoding according to the presentinvention as illustrated in FIG. 5 is a base station.

FIG. 6 illustrates a block diagram of a method for precoding accordingto embodiments of the present invention. It should be noted that stepsin FIG. 6 may be performed by the corresponding means in FIG. 5,respectively.

In step 601, a concatenated precoding matrix is constructed according toindices of a plurality of optimal precoding matrixes received from amobile terminal.

In an embodiment, precoding matrixes corresponding to the indices of theplurality of optimal precoding matrixes are retrieved in a codebook. Forexample, in the case of 2 antenna groups, indices of 2 optimal precodingmatrixes are received from the mobile terminal. Then, the corresponding2 precoding matrixes are retrieved in a codebook stored in a basestation.

Next, the plurality of retrieved precoding matrixes are combined into aconcatenated precoding matrix. In an embodiment, the concatenatedprecoding matrix obtained from respective precoding matrixes is achievedby directly combining the respective precoding matrixes into a largeprecoding matrix. For example, when there are 2 antenna groups,precoding matrixes corresponding to the antenna groups also total 2,e.g., represented by Ŵ¹ and Ŵ² respectively, where

${{\hat{W}}^{1} = \begin{bmatrix}{\hat{w}}_{1}^{1} \\{\hat{w}}_{2}^{1} \\\vdots \\{\hat{w}}_{n}^{1}\end{bmatrix}},{{\hat{W}}^{2} = \begin{bmatrix}{\hat{w}}_{1}^{2} \\{\hat{w}}_{2}^{2} \\\vdots \\{\hat{w}}_{m}^{2}\end{bmatrix}},$

and the combination of Ŵ¹ and Ŵ² is

$\overset{\sim}{W} = {\begin{bmatrix}{\hat{w}}_{1}^{1} \\{\hat{w}}_{2}^{1} \\\vdots \\{\hat{w}}_{n}^{1} \\{\hat{w}}_{1}^{2} \\{\hat{w}}_{2}^{2} \\\vdots \\{\hat{w}}_{m}^{2}\end{bmatrix}.}$

{tilde over (W)} shown above is the concatenated precoding matrixobtained according to Ŵ¹ and Ŵ².

In an embodiment, the plurality of retrieved precoding matrixes areweighted respectively, and then the plurality of weighted precodingmatrixes are combined into a concatenated precoding matrix. For example,the concatenated precoding matrix {tilde over (W)} is weighted by aweighting coefficient √{square root over (2)}/2 to satisfy thenormalization requirement, as shown by the equation:

$\overset{\sim}{W} = {\begin{bmatrix}{\hat{W}}^{1} \\{\hat{W}}^{2}\end{bmatrix} \cdot {\frac{\sqrt{2}}{2}.}}$

In another embodiment, the concatenated precoding matrix is weighted,e.g., phase and/or amplitude weighted, to satisfy the normalizationrequirement, as shown by the equation:

${\overset{\sim}{W} = {\begin{bmatrix}{{\cos(\theta)} \cdot {\hat{W}}^{1}} \\{{\mathbb{e}}^{j\varphi} \cdot {\sin(\theta)} \cdot {\hat{W}}^{2}}\end{bmatrix} \cdot \frac{\sqrt{2}}{2}}},$

where,

${\theta \in \left( {0,\frac{\pi}{2}} \right)},$φε(0,2π). θ and φ could be quantized at the mobile station according toa certain optimization criteria (e.g., maximizing SINR received at themobile station). For example, values are taken from value ranges of θand φ according to uniform quantization. Ŵ¹ and Ŵ² are weighted bydifferent values taken within the value ranges of θ and φ, so that theconcatenated precoding matrix {tilde over (W)} is determined moreflexibly.

In step 602, data is precoded using the constructed concatenatedprecoding matrix.

Data may be precoded with the concatenated precoding matrix as accordingto precoding methods in the prior art. For example, the precoding isimplemented using ZF (zero-forcing) criteria (see R1-071510, ‘Details ofZero-forcing MU-MIMO for DL EUTRA’, Freescale Semiconductor Inc., 3GPPTSG RAN WG1 #48bis, March 2007). Those skilled in the art may implementthe preceding according to any other available prior art, which is notdetailed here.

In step 603, the precoded data is sent to the mobile terminal.

FIG. 7 illustrates a comparison between simulation results of theconcatenated precoding according to the present invention, idealprecoding and existing precoding technology. In FIG. 7, the horizontalordinate denotes signal-to-noise ratio (SNR), and the vertical ordinatedenotes capacity, which is for measuring the effectiveness of a system.Capacity, with the unit of bps/Hz, is defined as the number oftransmissible bits per second over a unit-bandwidth transmissionchannel. Capacity is a metric of the amount of data passing at the unitbandwidth and thus measures the utilization efficiency of bandwidthresources by a signal transmission technique.

In simulations illustrated in FIG. 7, assume the method of the presentinvention adopts a single-BS MIMO system with the base station having 4transmit antennas, a mobile terminal having 2 receive antennas, 2 datastreams transmission with an MMSE detector, and a spatially uncorrelatedRayleigh flat fading channel. In FIG. 7 a comparison is made between thefollowing three precoding schemes:

Scheme 1: precoding based on ideal Channel State Information (CSI) at atransmitter. In this case, the transmitter will perform optimalprecoding based on traditional singular value decomposition (SVD).

Scheme 2: precoding based on a 4-bit DFT based codebook according to theprior art.

Scheme 3: concatenated precoding according to the present invention.

As is clear from FIG. 7, on the premise of the same signal-to-noiseratio (SNR), the capacity of the concatenated precoding method (3) ofthe present invention is closer to the ideal case (1) than the 4-bit DFTprecoding (2) of the prior art. This mainly comes from the betterdistance property of the concatenated precoding method of the presentinvention than 4-bit DFT precoding of the prior art.

The analysis result shown in Table 1 provides support to this. In table1, a comparison is made between the minimum Chordal distance, projectiontwo-norm distance and Fubini-Study distance of the concatenatedprecoding method of the present invention and the 4-hit DFT precoding ofthe prior art for rate 1 and rate 2 transmission. Rate 1 and rate 2 areaimed at different data streams, where rate 1 represents transmission ofone data stream while rate 2 represents transmission of two datastreams. These distances are effective metrics to evaluate if thestructure of a precoding matrix is good or not. The larger the minimumdistance, the better the codebook.

TABLE 1 Rate 1 Rate 2 Method of Method of the present the presentinvention Prior art invention Prior art Chordal distance 1.0707 1.00600.8040 0.1199 Projection two-norm 0.3827 0.1095 0.7071 0.1182 distanceFubini-Study distance 0.3927 0.3927 0.8589 0.1202

FIG. 8 illustrates a comparison between simulation results of theconcatenated precoding according to the present invention, phase and/oramplitude weighted concatenated precoding according to the presentinvention, ideal precoding, and existing precoding technology. In FIG.8, the horizontal ordinate denotes signal-to-noise ratio, and thevertical ordinate denotes capacity.

As is clear from FIG. 8, on the premise of the same signal-to-noiseratio (SNR), the concatenated precoding method (2) according to thepresent invention and phase and/or amplitude weighted concatenatedprecoding (3) according to the present invention have larger capacitythan 4-bit DFT precoding (4) of the prior art. And compared with theunweighted concatenated precoding method (2), weighted concatenatedprecoding (3) is closer to the ideal case (1).

The present invention provides a new and effective codebook constructionmethod whereby a large concatenated precoding matrix is obtained byconcatenating smaller precoding matrixes in the codebook. The method ofthe present invention is especially applicable to a multi-BS MIMO systemand a single-BS MIMO system with a large number of transmit antennas.The codebook constructed by the method of the present invention has gooddistance property in terms of the Chordal distance, the projectiontwo-norm distance and the Fubini-Study distance Chordal distance.Moreover, the method according to the present invention does notincrease interference in the communication system, because concatenationof precoding matrixes of the present invention does not result ininterference uncertainty issue.

The present invention is suitable for a multi-BS MIMO system because thecodebook construction is not relevant to the BS size involved inmulti-BS coordination and is flexible enough to adapt to differentchanges in multi-BS coordination. Moreover, the present invention hasother advantages like constant tnodulo, nested structure, backwardcompatibility, high scalability, etc.

It should be pointed out that the methods as disclosed in the presentinvention can be implemented in software, hardware or combination ofsoftware and hardware. The hardware portion can be implemented by usingdedicated logic; the software portion can be stored in memory andexecuted by an appropriate instruction executing system such as amicroprocessor, a personal computer (PC) or a mainframe computer.

The specification of the present invention has been presented forpurposes of illustration and description, and is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art.

Therefore, the embodiments were chosen and described in order to bestexplain the principles of the invention and the practical application,and to enable others of ordinary skill in the art to understand that allmodifications and alterations made without departing from the spirit ofthe present invention fall into the protection scope of the presentinvention as defined in the appended claims.

What is claimed is:
 1. A method for constructing a codebook, comprising:receiving grouping information on a plurality of transmit antennas of abase station; obtaining a channel model for a channel between a mobileterminal and the base station; and determining an optimal precodingmatrix corresponding to each antenna group according to the groupinginformation and the channel model, so as to construct a codebook usedfor precoding, wherein the determining comprises: applying the groupinginformation and the channel model to predetermined criteria, so as todetermine an optimal precoding matrix that satisfies the predeterminedcriteria; calculating, according to the grouping information and thechannel model, a precoding matrix that makes a Chordal distance, aprojection two-norm distance or a Fubini-Study distance be minimum; anddetermining the precoding matrix that makes the distance be minimum asthe optimal precoding matrix.
 2. An apparatus for constructing acodebook, comprising: a receiver for receiving grouping information on aplurality of transmit antennas of a base station; a channel modelobtaining module for obtaining a channel model for a channel between amobile terminal and the base station; and a precoding matrix determiningmodule for determining an optimal precoding matrix corresponding to eachantenna group according to the grouping information and the channelmodel, so as to construct a codebook used for precoding, wherein thedetermining comprises: applying the grouping information and the channelmodel to predetermined criteria, so as to determine an optimal precodingmatrix that satisfies the predetermined criteria; calculating, accordingto the grouping information and the channel model, a precoding matrixthat makes a Chordal distance, a projection two-norm distance or aFubini-Study distance be minimum; and determining the precoding matrixthat makes the distance be minimum as the optimal precoding matrix.
 3. Amethod for selecting a precoding matrix, comprising: receiving groupinginformation on a plurality of transmit antennas of a base station;estimating a channel from the base station to obtain a channel model;selecting from a codebook an optimal precoding matrix corresponding toeach antenna group, based on the grouping information and the channelmodel, wherein the selecting comprises: applying the groupinginformation and the channel model to predetermined criteria, so as todetermine a precoding matrix that satisfies the predetermined criteria;calculating, according to the grouping information and the channelmodel, a precoding matrix that makes a Chordal distance, a projectiontwo-norm distance or a Fubini-Study distance be minimum; and determiningthe precoding matrix that makes the distance be minimum as the optimalprecoding matrix; and sending an index of each optimal precoding matrixto the base station so that the base station performs concatenatedprecoding.
 4. The method according to claim 3, wherein selecting theoptimal precoding matrix further comprises: retrieving in the codebook aprecoding matrix matching the precoding matrix that satisfies thepredetermined criteria, as the optimal precoding matrix.
 5. An apparatusfor selecting a precoding matrix, comprising: a receiver for receivinggrouping information on a plurality of transmit antennas of a basestation; a channel model obtaining module for estimating a channel fromthe base station to obtain a channel model; a precoding matrix selectingmodule for selecting from a codebook an optimal precoding matrixcorresponding to each antenna group, based on the grouping informationand the channel model, wherein selecting the optimal precoding matrixfurther comprises: calculating, according to the grouping informationand the channel model, a precoding matrix that makes a Chordal distance,a projection two-norm distance or a Fubini-Study distance be minimum;and determining the precoding matrix that makes the distance be minimumas the optimal precoding matrix; and a transmitter for sending an indexof each optimal precoding matrix to the base station so that the basestation performs concatenated precoding.
 6. The apparatus according toclaim 5, wherein the precoding matrix selecting module is configured to:apply the grouping information and the channel model to predeterminedcriteria, so as to determine a precoding matrix that satisfies thepredetermined criteria; and retrieve in the codebook a precoding matrixmatching the precoding matrix that satisfies the predetermined criteria,as the optimal precoding matrix.
 7. A communication system, comprising:an apparatus according to claim 5; and an apparatus comprising aconstructing module for constructing a concatenated precoding matrixaccording to indices of a plurality of optimal precoding matrixesreceived from a mobile terminal, a precoding module for precoding datausing the constructed concatenated precoding matrix, and a transmitterfor sending the precoded data to the mobile terminal.
 8. A method forprecoding, comprising: constructing a concatenated precoding matrixaccording to indices of a plurality of optimal precoding matrixesreceived from a mobile terminal, wherein the constructing comprises:retrieving in a codebook precoding matrixes that correspond to theindices of the plurality of optimal precoding matrixes; and combiningthe retrieved precoding matrixes into the concatenated precoding matrix;precoding data using the constructed concatenated precoding matrix; andsending the precoded data to the mobile terminal.
 9. The methodaccording to claim 8, wherein the combining the retrieved precodingmatrixes into the concatenated precoding matrix further comprises:weighting the retrieved precoding matrixes, respectively; and combiningthe weighted precoding matrixes into the concatenated precoding matrix.10. The method according to claim 8, wherein the combining the retrievedprecoding matrixes into the concatenated precoding matrix furthercomprises: weighting the concatenated precoding matrix so as to achievenormalization.
 11. An apparatus for precoding, comprising: aconstructing module for constructing a concatenated precoding matrixaccording to indices of a plurality of optimal precoding matrixesreceived from a mobile terminal, wherein the constructing module isconfigured to: retrieve in a codebook precoding matrixes that correspondto the indices of the plurality of optimal precoding matrixes; andcombine the retrieved precoding matrixes into the concatenated precodingmatrix; a precoding module for precoding data using the constructedconcatenated precoding matrix; and a transmitter for sending theprecoded data to the mobile terminal.